1. Field of the Invention
The present invention relates in general to a ceramic envelope device for use in a high-pressure discharge lamp (hereinafter referred to as "HID lamp"; "HID" representing High Intensity Discharge), and more particularly to the structure of one or both ends of such a ceramic envelope device at which a translucent ceramic tube of the envelope device is closed by an electrically conductive end cap and sealed by a sealing member provided between the end cap and the ceramic tube.
2. Discussion of the Prior Art
In the art of such HID lamps using a translucent ceramic tube, a pair of electrically conductive discs are known as end caps to close the longitudinally opposite open ends of the translucent ceramic tube. Examples of such closure end caps are illustrated in U.S. Pat. Nos. 4,155,757 and 4,155,758. Such end caps are formed of an electrically conductive cermet obtained by mixing, for example, particles of tungsten with particles of aluminum oxide, and sintering the mixture. These electrically conductive cermet end caps support at their inner surfaces, a pair of discharge electrodes of tungsten so that the electrodes protrude from the respective inner surfaces of the end caps toward each other, i.e., longitudinally inward in the translucent ceramic tube. Also, electrical contact rods or lead rods are connected or fixed to the outer surfaces of the cermet end caps by suitable methods, so that electric power is applied to the pair of oppositely located tungsten electrodes through the contact rods and through the cermet end caps.
Such cermet end caps have been advantageously employed, for example, in high-pressure sodium lamps, because they eliminate the need of using expensive metallic niobium. It is further recognized that such cermet end caps have been advantageously used for so-called metal halide lamps which employ a translucent ceramic tube charged with a suitable metal halide for improved color-rendering, as well as with mercury and rare gas, because the cermet exhibits relatively high corrosion resistance to metal halides.
However, such a HID lamp with a translucent ceramic tube closed by cermet end caps may suffer from a problem generally known as "arc-back" phenomenon wherein an arc will take place between the discharge electrodes and the corresponding cermet end caps, rather than between the oppositely located electrodes, when the HID lamp is initially turned on. This arc-back phenomenon causes the cermet end caps to crack, thereby causing the ceramic envelope device to leak. In addition, the "arc-back" phenomenon gives rise to vaporization and scattering of the refractory metal components in the cermet, and consequent deposition thereof on the inner surface of the ceramic tube, which results in blackening of the wall of the ceramic tube, thereby reducing the luminous flux of the envelope device. For overcoming this problem, it has been recommended in the art that an electrical insulator in the form of an insulating layer be provided on the inner surfaces of the cermet end caps from which the electrodes protrude.
As to the metal halide lamp indicated above, it is also recognized that supersaturated metal halide in the ceramic tube may condense at the cold spot in the ceramic tube (i.e., at the lower end portion of the ceramic tube disposed vertically when the lamp is used in its upright position), whereby a sealing member provided for sealing the lower end of the ceramic tube is subjected to corrosion due to the liquid phase of the condensed metal halide and the heat generated by the lower electrode in use, with a result of deterioration of the lamp properties and/or causing the ceramic envelope device to leak. For overcoming these problems, it has been practiced in the art that the lower end portion of the ceramic tube be gas-tightly closed or sealed by a shrinkage-fit of a cermet end cap, without using any sealing means. In this case, the upper end of the ceramic tube is closed by a cermet end cap and sealed by a sealing member, after charging of the ceramic tube with a suitable metal halide through the upper end thereof.
In the above type of ceramic envelope device, at least one of the opposite ends of which is sealed by a sealing member or material, it is required to completely fill a gap between the ceramic tube and the at least one end cap with the sealing member. Consequently, the sealing member is partially exposed in an inner space of the ceramic tube in which the metal halide vapor is present, and the sealing member is subjected to corrosion due to the metal halide, though that corrosion might not be so severe as that of a sealing member located at the lower end of an envelope device of an upright-oriented lamp. Therefore, the HID lamp incorporating such a ceramic envelope is not free from the problems of the leakage of the envelope device or deteriorated operating characteristics such as lowered luminous flux (blackening of the wall of the ceramic tube) or change of color, temperature and lamp voltage.
Additionally, where the sealing between the ceramic tube and the cermet end cap with the sealing member is insufficient, that is, a side surface of the cermet end cap is not completely covered by the sealing member, the arc-back phenomenon may take place at the non-sealed portion of the cermet end cap, thereby causing the end cap, sealing member and/or ceramic tube to crack which in turn may cause leakage of the charged gases from the envelope device.